A sealing labyrinth, also known as a labyrinth seal, comprises a rotary portion having fins (or wipers) and a static bore covered in a soft abradable material or in a honeycomb structure capable of withstanding high temperatures. When the engine starts, the fins rub lightly against the lining, biting into it, thus leading to minimum spacing. This clearance varies over various cycles in flight, depending on the expansion of the part and on the natural flexibility of the moving portions.
The wipers of labyrinths serve to provide aerodynamic sealing between enclosures containing air at different pressures. They are generally situated on the rotor portion facing stator portions. They are mainly constituted by continuous or segmented “blades” of annular shape, capable of being directed radially inwards or outwards.
In particular, when they are continuous in shape, wipers are liable to come into contact with the stator in certain operating configurations. In order to avoid them being destroyed in such situations, stators are fitted with coatings that provide the interface and that are referred to as being “abradable”. Under such circumstances, the usual sequences for wiper penetration into the abradable layer consist in a radial cut associated with an axial displacement (“straight turning”).
In reality, the usual abradable materials can turn out to be relatively abrasive, in particular against certain continuous wipers, particularly if they are made of titanium-based alloy, but also if they are made of steel or of nickel-based alloy. This is particularly true when the abradable linings are made in the form of honeycombs of refractory alloy.
In order to avoid wipers being damaged, or even destroyed, they are commonly coated by thermally spraying (plasma torch, high velocity oxyfuel (HVOF), . . . ) an abrasive deposit of the alumina and titanium dioxide or carbide type, e.g. on an underlayer of aluminum and nickel alloy in order to provide bonding.
Deposition by thermal spraying requires relative spray angles to be complied with between the axis of the torch and the surfaces of the parts to be coated, so that the sprayed particles impact as close as possible to orthogonally on the surface for coating in order to obtain a deposit of satisfactory quality and with satisfactory bonding. That technique also requires a minimum distance between the spraying tool and the surface: the hot central zone of the flame from the torch can be at several thousands of degrees centigrade, so it must be kept far enough away from the part; in addition, the particles to be deposited must be accelerated sufficiently to adhere on the surfaces that are to be protected.
Furthermore, the propellant or plasma-generating gases used for spraying purposes must be easy to exhaust while ensuring that the sprayed powder is not “blown about” by creating turbulence.
In general, the wipers are oriented practically orthogonally to the cylindrical surfaces of the rotors, and they are often situated close to disk or labyrinth sheets at the bottoms of cavities or close to other wipers when they are placed in series.
Amongst these situations, numerous circumstances arise that present a geometrical arrangement that makes deposition by thermal spraying very uncertain or practically impossible.